Evidence of internal intergranular oxidation as a mechanism of primary water stress corrosion cracking in alloy 600

Alloy 600 specimens were exposed to a steam-hydrogen environment considered to simulate exposure to primary water conditions in nuclear power plants. Ni nodules were expelled to the surface indicating the likelihood of internal oxidation. A cross-section of the exposed specimen was prepared using a focused ion beam and a composition profile across the cross-section was generated to confirm internal oxidation of Cr. It was revealed that internal intergranular oxidation occurred, and embrittlement was confirmed by the appearance of cracks in the thin section. From these results, it can be concluded that in primary water-type environments Alloy 600 can undergo internal intergranular oxidation of Cr causing embrittlement of grain boundaries.